Here is a nifty little stainless steel coolant recovery tank to fit in tight spaces. This one is 2-inches diameter and 10-inches tall. That should hold slightly more than one pint of fluid, which may be just enough to satisfy a 6-quart cooling system. Tucked right up in the corner near the radiator it may not interfere too much with other maintenance.

The screw cap is o-ring sealed to prevent any top spill. The can is vented from below with small bore tubing. The fluid pass tube connects at the bottom of the tank. The vent tube goes through the bottom and extends internally near the top. No site glass here, so you would have to remove the cap to check fluid level, but the canister is not pressurized, so no problem checking either hot or cold. Total cost less than $25; search for "stainless steel overflow tank" on eBay. =
http://www.ebay.com/sch/i.html?_nkw=%22stainless+steel+overflow+tank%22